node-jpegoptim

Optimize JPEGs in memory

What

Losslessly transform jpegs to save some bytes. This is primarily done by telling libjpeg to optimize huffman codes, and by throwing away some garabge segments that another software might have inserted.

Also allows to strip metadata (EXIF, IPTC, XMP) segments and/or ICC profile segments (tho your colors will look bigly wrong).

Requirements

• Node, obviously, a recent version (at the time of writing, 10 and 11 tested).
• Linux, macOS, potentially other *nix (untested)
• x86_64, potentially others (untested)
• libjpeg (or libjpeg-turbo or libmozjpeg, whatever pkg-config finds as libjpeg)
• Optionally libexif to enable stripThumbnail.

E.g. to build it on macOS with mozjpeg and libexif from brew, do:

export PKG_CONFIG_PATH=$(brew --prefix mozjpeg)/lib/pkgconfig:$(brew --prefix libexif)/lib/pkgconfig
yarn add @dolos/jpegoptim

How

The API is really simple: there is only one function (default export of the module) and one custom error type.

jpegoptim(buf, [options])

• @param {Buffer} buf Buffer containing the JPEG to optimize
• @param {Object} [options] Some options for you.
• @param {Buffer|Number} [options.out] Either the output buffer to use. The result is the used slice. Or a number limitting the buffer size. If omitted, a new Buffer without upper bound for the size is returned. Please remember that this function might add JFIF headers in otherwise already optimized files, which can lead to slight growing in size.
• @param {Boolean} [options.strip] Strip all meta data.
• @param {Boolean} [options.stripICC] Strip all ICC profile data.
• @param {Boolean} [options.stripThumbnail] Strip any EXIF thumbnail present in the image metadata. Requires that this module was compiled against libexif.
• @returns {Promise<Buffer>} The optimized jpeg.
• @throws TypeError
• @throws RangeError
• @throws OptimizeError

Moreover, there is a feature to dump the raw dct stream of an image. This allows to e.g. compare image data quickly without the need for full decoding, i.e. two images, e.g. one original and one losslessly optimized should still yield the same DCT stream.

jpegoptim.dumpdct(buf, func)

• @param {Buffer} buf Buffer containing the JPEG to dump
• @param {Function} func callback function receiving the raw dct buffers. The callback function will be called multiple times, usually with a line of blocks. Please note that the buffer func will receive will be reused and thus change between calls. As such, you must copy the buffer if you want to keep the contents. Callback invocations happen only before dumpdct returns. The function is synchronous.
• @throws TypeError
• @throws RangeError
• @throws OptimizeError

Additionally jpegoptim has the following properties

• @property {OptimizeError} OptimizeError Reference to OptimizeError

• @property {Object} versions Library version of libjpeg etc

• @property {Boolean} supportsThumbnailStripping Does this build support it?

See sample.js for a small program demonstrating the use.

Design

• Uses whatever your system libjpeg is (or what pkg-config said it was).
• To provide EXIF thumbnail stripping functionality, uses whatever libexif is on your system (if any). If there is none, that feature will not be available.
• Offload to the node worker pool.
• Avoid buffer memory copies. Operate directly on the input buffer. And either create an (external) output buffer, or operate directly on the user supplied output buffer.

Todo

(as in: probably won't do)

• Support Windows
• Support systems where libjpeg cannot be pkg-config'ed
• Support node versions I am not using.